1. Field of the Invention
The present invention relates to a technique for mounting a surface mount element on a circuit board. In particular, the present invention relates to an electronic component having a surface mount element mounted on a circuit board using an ultrasonic welding method, a method for manufacturing the electronic component, and a circuit board.
2. Description of the Related Art
A flexible multilayer resin substrate may be used as a circuit board. For example, such a circuit board having a multilayer structure is formed by thermally softening and press-bonding a plurality of layers of thermoplastic resin, such as liquid crystal polymer resin.
Examples of methods for mounting a surface mount element on such a circuit board include an ultrasonic welding method (see, e.g., Japanese Unexamined Patent Application Publication No. 2006-120683). FIG. 7 is a schematic diagram illustrating an ultrasonic welding method described in Japanese Unexamined Patent Application Publication No. 2006-120683.
The ultrasonic welding method is a technique in which terminal electrodes (not shown) of a surface mount element 103 are welded to mount electrodes (not shown) of a circuit board 101 by frictional heat generated on the contact surfaces by ultrasonically vibrating the surface mount element 103 which is in contact with the circuit board 101. In the technique described in Japanese Unexamined Patent Application Publication No. 2006-120683, a liquid crystal polymer resin is adopted as a material of the circuit board 101. The liquid crystal polymer resin has a crystal orientation, and its elastic modulus along the crystal orientation is greater than those along other orientations. Therefore, if the direction in which the surface mount element 103 is ultrasonically vibrated is not along the crystal orientation of the liquid crystal polymer resin in the circuit board 101, the ultrasonic vibration may be absorbed in the circuit board 101 and the bonding strength of the surface mount element 103 may be lowered. In the technique described in Japanese Unexamined Patent Application Publication No. 2006-120683, the direction in which the surface mount element 103 is ultrasonically vibrated is aligned with the crystal orientation of the liquid crystal polymer resin in the circuit board 101, so as to stabilize the bonding strength of the surface mount element 103 to the circuit board 101.
In the ultrasonic welding method, as described above, the surface mount element may not be bonded to the circuit board with a sufficient bonding strength, due to absorption of ultrasonic vibration in the circuit board. In particular, if the circuit board is made of a soft material, such as liquid crystal polymer, the vibrational energy of the ultrasonic vibration is not easily transmitted and it is not easy to stabilize the bonding. As a result, for example, if the circuit board is bent, the surface mount element may fall off the circuit board or poor contact of the surface mount element may occur.
In particular, if the circuit board is made of a thermoplastic material, such as liquid crystal polymer, frictional heat may locally soften the circuit board and may cause the surface of the circuit board to cave in. Thus, sufficient pressure may not be applied to the surface mount element, and the bonding strength may be weakened. To stably achieve a sufficient bonding strength when the circuit board is made of a material having a crystal orientation, it is necessary to determine the crystal orientation in the circuit board and appropriately set the direction of ultrasonic vibration. This may increase the degree of difficulty of the mounting process.